Filter media with charge stabilizing and enhancing additives

ABSTRACT

Electret filter media having an increased electrostatic charge that is substantially maintained in the presence of heat are provided. In one exemplary embodiment, the filter media includes a melt processable charge enhancing additive, such as a fatty acid amide, and a melt processable charge stabilizing additive, such as a fatty acid metal salt. The charge enhancing additive is particularly effective to increase or enhance the electrostatic charge of the filter media when a charge is imparted thereto, and the charge stabilizing additive is particularly effective to stabilize the charge such that, when the filter media is subjected to a heat treatment, the enhanced electrostatic charge is substantially maintained.

FIELD OF THE INVENTION

The present invention relates to electret filter media having an increased level of electrostatic charge that is substantially maintained in the presence of heat.

BACKGROUND OF THE INVENTION

Electret filter media have long been used in many filtration applications. Electret filter media are those that include a dielectric insulating polymer web that is treated to possess substantially permanent spatially oriented, opposite charge pairs or dipoles. Among the common polymer webs used for electret filter media are polypropylene, polyethylene, polyester, polyamide, polyvinyl chloride, and polymethyl methylacrylate.

Conventional filter media are substantially lacking in electrostatic charge and rely upon impingement, impaction and diffusion for filter performance. Electret filter materials offer improved filtering performance over conventional filter materials. The presence of oriented dipoles in the electret filter media is believed to enhance filter performance by allowing the filter media to attract and retain charged and uncharged particles to be filtered.

Electret filter materials are made by a variety of known techniques. One technique for manufacturing electret filter media involves extruding a polymer, typically having a high melt flow index, through a die having a linear array of orifices. An air knife is used to attenuate the extruded polymer fibers by a ratio of about 300:1. The attenuated fibers are then collected on a rotating drum or moving belt using a moderate vacuum. The fiber web is then treated to impart on the fiber web charge pairs or dipoles. The charge pairs or dipoles can be imparted to the fiber, for example, using AC and/or DC corona discharge.

One problem associated with electret filter material is that the charge pairs or dipoles imparted to the filter media often are not stable. In some instances, charge or its spatial orientation is lost after filtering certain contaminants for relatively short time periods. The result is a marked decrease in filter performance over a relatively short period of time (e.g., less than 20 minutes). One other problem associated with electret filter material is their inability to maintain the electrostatic charge after being subjected to heat. Manufacturing standards for respiratory products, for example, often mandate that final respiratory mask be subjected to a thermal treatment process to simulate an aged phenomenon.

Accordingly, there exists a need for electret filter media having an increased electrostatic charge in combination with an enhanced charge stability.

SUMMARY OF THE INVENTION

The present invention provides

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides electret filter media having an increased electrostatic charge that is substantially maintained in the presence of heat. In one exemplary embodiment, the filter media includes a melt processable charge enhancing additive, such as a fatty acid amide, and a melt processable charge stabilizing additive, such as a fatty acid metal salt. The charge enhancing additive is particularly effective to increase or enhance the electrostatic charge of the filter media when a charge is imparted thereto, and the charge stabilizing additive is particularly effective to stabilize the charge such that, when the filter media is subjected to a heat treatment, the enhanced electrostatic charge is substantially maintained. This is particularly desirable for use in respiratory applications, however the filter media can be adapted for use in a variety of applications including, by way of non-limiting example, ASHRAE filters, vacuum bag filters, vacuum exhaust filters, room air cleaner filters, engine/cabin air filters, HEPA (High Efficiency Particulate Air) filters, and ULPA (Ultra Efficiency Particulate Air) filters.

The meltblown polymer fiber web can be formed from a variety of polymeric materials, which may vary depending on the intended use. By way of non-limiting example, suitable polymers include polyethylene, polypropylene, polyamide, polyvinyl chloride, polymethylmethacrylate, polyester, and mixtures thereof.

The meltblown polymer fiber web can also include a variety of melt processable charge enhancing additives. The charge enhancing additive can be, for example, a fatty acid amide that is derived from a fatty acid, which includes saturated or unsaturated straight chain carboxylic acids obtained from the hydrolysis of fats. Exemplary fatty acids include lauric acid (dodecanoic acid), myristic acid (tetradecanoic acid), palmitic acid (hexadecanoic acid), stearic acid (octadecanoic acid), oleic acid ((Z)-9-octadecenoic acid), linoleic acid ((Z,Z)-9,12-octadecadienoic acid), linolenic acid ((Z,Z,Z)-9,12,15-octadecatrienoic acid) and eleostearic acid (Z,E,E)-9,11,13-octadecatrienoic acid). Typically, the amides formed from the above referenced acids are primary amides which are prepared by methods well known in the art.

Secondary and tertiary fatty acid amides are also suitable as charge enhancing agents wherein the amide nitrogen is substituted with one or more alkyl groups. Secondary and tertiary fatty acid amides can also be prepared by methods well known in the art, such as by esterification of a fatty acid followed by an amidation reaction with a suitable alkylamine. The alkyl substituents on the amide nitrogen can be straight chain or branched chain alkyl groups and can have between about two and twenty carbon atoms, inclusive, preferably between about two and 14 carbon atoms, inclusive, more preferably between about two and six carbon atoms, inclusive, most preferably about two carbon atoms. In one exemplary embodiment, the fatty acid amide can be a “bis” amide wherein an alkyl chain tethers two nitrogens of two independent amide molecules. For example, alkylene bis-fatty acid amides include alkylene bis-stearamides, alkylene bis-palmitamides, alkylene bis-myristamides and alkylene bis-lauramides. Typically the alkyl chain tether includes between about 2 and 8 carbon atoms, inclusive, preferably 2 carbon atoms. The alkyl chain tether can be branched or unbranched. Preferred bis fatty acid amides include ethylene bis-stearamides, such as ACRAWAX™ C, available from Lonza, Inc. of Fair Lawn, N.J., and ethylene bis-palmitamides such as N,N′-ethylenebistearamide and N,N′-ethylenebispalmitamide.

One type of useful charge enhancing additive, as noted above, are fatty acid amides. Examples of exemplary fatty acid amides include stearamide and ethylene bis-stearamide. An exemplary stearamide is commercially available as UNIWAX 1750, available from UniChema Chemicals, Inc. of Chicago, Ill. ACRAWAX® C is an ethylene bis-stearamide which is commercially available from Lonza, Inc. of Fair Lawn, N.J. ACRAWAX® C contains N,N′-ethylenebissteramide (CAS No. 110-30-5) and N,N′-ethylenebispalmitamide (CAS No. 5518-18-3) with a mixture of C-14 to C-18 fatty acid derivatives (CAS No. 67701-O₂-4) with an approximate ratio of 65/35/2 (N,N′-ethylenebissteramide/N,N′-ethylenebispalmitamide/mixture of C-14 to C-18 fatty acid derivatives) by weight. For example, the commercial product includes N,N′-ethylenebisstearamide, N,N′-ethylenebispalmitamide with C14-C18 fatty acids. In certain embodiments of the invention, either N,N′-ethylenebisstearamide or N,N′-ethylenebispalmitamide can be the sole charge enhancing additive. In another embodiment, the ratio of a C14-C18 fatty acid can be varied from between about 0 to 20% based on the total amount of the bisamides. In still other embodiments, mixtures of N,N′-ethylenebisstearamide and N,N′-ethylenebispalmitamide which fall in the range between about 0 to 100% for each bisamide can be utilized as additive mixtures, e.g., 80/20, 70/30, 5/50, etc.

The meltblown polymer fiber web can also include a charge stabilizing additive, such as a fatty acid metal salt, which is effective to stabilize the electrostatic charge in the web, particular when the fiber web is subjected to heat. While virtually any fatty acid metal salt can be used, the fatty acid portion of the fatty acid metal salt can be, for example, lauric acid, palmitic acid, stearic acid, oleic acid, etc., and the metal portion of the fatty acid metal salt can be, for example, magnesium, zinc, aluminum, etc. In an exemplary embodiment, the fatty acid metal salt is zinc stearate or magnesium stearate.

The meltblown polymer fiber web can be formed using a variety of techniques, but in one exemplary embodiment the charge enhancing additive, e.g., a fatty acid amide, and the charge stabilizing additive, e.g., a fatty acid metal salt, are mixed with a polymer resin to form a composition that is extruded into fibers to form a polymer fiber web. An exemplary process for forming a meltblown polymer fiber web is described in more detail in U.S. Pat. No. 6,780,226, which is incorporated herein by reference in its entirety.

A person skilled in the art will appreciate that the charge enhancing additive and the charge stabilizing additive can be combined with the polymer resin in a number of ways. In one example, the additives can be combined with the resin using a two screw extruder, yielding polymer pellets with a concentrated amount of each additive. These concentrated pellets, alone or combined with other polymer pellets, are then passed through an extrusion process that yields the desired polymer fiber web.

The concentration of each additive in the composition can vary depending on the intended use of the filter media. In one exemplary embodiment, the charge enhancing additive, such as a fatty acid amide, is present within the web at a concentration in the range of about 0.5% to 11% by weight, and more preferably about 1% to 8% by weight, and most preferably at about 1% by weight, and the charge stabilizing additive, such as a fatty acid metal salt, is present within the web at a concentration in the range of about 0.1% to 20% by weight, and more preferably about 0.1% to 5% by weight, and most preferably at about 0.4% by weight.

The resulting polymer fiber web that is formed from extruding the composition, e.g., the polymer pellets, can be comprised of fibers having a relatively broad distribution of fiber diameters. In one exemplary embodiment, the average fiber diameter can be in the range of about 1μ to 15μ, and more preferably about 3μ. The basis weight of the polymer fiber web can also vary, especially considering the intended application. In general, higher web basis weights yield better filtration, but there exists a higher resistance, or pressure drop, across the filter barrier when the filter media has a higher basis weight. For most applications, the basis weight can be in the range of about 110 g/m² to 200 g/m², and more preferably from about 20 g/m² to 70 g/m². Exemplary basis weights include 20 g/m², 40 g/m², and 60 g/m². One of ordinary skill in the art can readily determine the optimal web basis weight, considering such factors as the desired filter efficiency and permissible levels of resistance. Furthermore, the number of plies of the polymer fiber web used in any given filter application can also vary. One of ordinary skill in the art can readily determine the optimal number of plies to be used.

Once the meltblown polymer fiber web is formed, an electrostatic charge can be imparted to the web to form an electret polymer fiber web. A variety of techniques are well known to impart a permanent dipole to the polymer web in order to form electret filter media. Charging can be effected through the use of AC and/or DC corona discharge units and combinations thereof. The particular characteristics of the discharge are determined by the shape of the electrodes, the polarity, the size of the gap, and the gas or gas mixture. In one embodiment charging can be accomplished solely through the use of an AC corona discharge unit. In another embodiment it is useful to use both AC and DC corona discharge units. In a preferred technique the polymer web is first subjected to AC corona discharge followed by one or more successive treatments by a DC corona discharge unit. Charging can also be accomplished using other techniques, including friction-based charging techniques. Typically the fiber web is subjected to a discharge of between about 1 to about 30 kV (energy type, e.g., DC discharge or AC discharge)/cm, inclusive, preferably between about 10 kV/cm and about 30 kV/cm, inclusive, with a preferred range of between about 10 to about 20 kV/cm, inclusive.

It will be appreciated by one skilled in the art that corona unit(s), AC corona discharge unit(s) and/or DC corona discharge unit(s) can be placed above and/or below a meltblown fiber web to impart electret properties to the fiber web. Configurations include placement of a neutrally grounded roll(s) on either side of the fiber web and the active electrode(s) above or below either side of the web. In certain embodiments, only one type of corona discharge unit, e.g., a DC or an AC corona discharge unit, is placed above, below or in an alternating arrangement above and below the fiber web. In other embodiments alternating AC or DC corona discharge units can be used in combination. The AC or DC corona discharge unit can be controlled so that only positive or negative ions are generated.

In one embodiment, a permanent dipole can be imparted to the polymer fiber web as follows. The web is first charged using an AC corona, followed by a charging with a series of DC corona discharge units, e.g., DC charge bars. The DC corona discharge units are positioned on alternating sides of the passing fiber web and each successive DC corona discharge unit applies a charge of a different polarity, i.e., positive/negative. In a preferred embodiment, the charge of the DC corona discharge units located above and below the nonwoven web alternates from positive to negative in a series of treatments, e.g., 2, 4, 6, etc. Alternatively, the DC corona discharge units are positive or negative and do not alternate in charge.

An exemplary process for producing electret properties in fiber webs can also be found in U.S. Pat. No. 5,401,446, the contents of which are incorporated herein by reference in its entirety.

In use, the meltblown electret polymer fiber web has an increased electrostatic charge and efficiency, due to the charge enhancing additive, and the electrostatic charge is substantially maintained when the web is subjected to heat, due to the charge stabilizing additive. While filter performance can be evaluated based on different criteria, it is desirable that filters, or filter media, be characterized by low penetration across the filter of contaminants to be filtered. At the same time, however, there should exist a relatively low pressure drop, or resistance, across the filter. Penetration, often expressed as a percentage, is defined as follows: Pen=C/C ₀

where C is the particle concentration after passage through the filter and C₀ is the particle concentration before passage through the filter. Filter efficiency is defined as 100−% Penetration.

Because it is desirable for effective filters to maintain values as low as possible for both penetration and pressure drop across the filter, filters are rated according to a value termed alpha (a), which is the slope of log penetration versus pressure drop across the filter. Steeper slopes, or higher alpha values, are indicative of better filter performance. Alpha is expressed according to the following formula α=−100 log (C/C ₀)/DP,

-   -   where D P is the pressure drop across the filter.

In many filtering situations it is important to have a high initial alpha value. However, it is equally, if not more important, to maintain acceptable alpha values well into the filtration process. In particular, in respiratory applications, manufacturing standards mandate that the final respiratory filter, such as a respiratory mask, be subjected to elevated temperatures to simulate an aged effect. Accordingly, the filter media must be capable of maintaining a high alpha value when subjected to heat.

Some standard tests for evaluating filter performance focus on penetration and resistance (as related by alpha value) after 200 milligrams of loading. Alpha decay is generally not a problem in filtering gases that contain only solids. In fact, in such filtering applications the alpha value often increases over time. The phenomenon of alpha decay is more evident while filtering gases that contain liquid droplets or a mixture of liquid droplets and solid particles.

While various standard tests can be used to evaluate filter performance, one exemplary test is the DOP (dioctyl phthalate) challenge, which employs an automated filter testing unit purchased from TSI, Inc. equipped with an oil generator. The instrument measures pressure drop across filter media and the resultant penetration value on an instantaneous or “loading” basis at a flow rate less than or equal to 115 liters per minute (1 pm). Instantaneous readings are defined as 1 pressure drop/penetration measurement. Another exemplary test is the NaCl (sodium chloride) challenge, which employs a CertiTest™ automated filter testing unit from TSI, Inc. equipped with a sodium chloride generator. The average particle size created by the unit is 0.3μ to 0.5μ. The instrument measures a pressure drop across the filter media and the resultant penetration value on an instantaneous basis at a flow rate less than or equal to 115 liters per minute (1 pm). Instantaneous readings are defined as 1 pressure drop/penetration measurement.

When determining the alpha value, based on the DOP and NaCl challenges, of a meltblown electret polymer fiber web in accordance with the present invention, the initial alpha value, i.e., the alpha value determined prior to subjecting the filter media to heat, is substantially maintained when the filter media is subjected to heat, e.g., a temperature of at least about 70° C.

The following non-limiting examples serve to further illustrate exemplary embodiments of the present invention:

EXAMPLE 1

Sample 1-1 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99% by weight of polypropylene and 1% by weight of ACRAWAX™ C. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 3.6μ, and a web basis weight of 23.69 g/m².

Sample 1-2 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99.6% by weight of polypropylene and 0.4% by weight of magnesium stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 3.51μ, and a web basis weight of 22.25 g/m².

Sample 1-3 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 98.6% by weight of polypropylene, 1% by weight of ACRAWAX™ C, and 0.4% by weight of magnesium stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 8μ, and a web basis weight of 22.2 g/m².

Table 1 illustrates the penetration, the resistance at 10.5 fpm face velocity, and the alpha value of Samples 1-1, 1-2, and 1-3, as tested using a NaCl challenge, initially and after subjecting the webs to a temperature of at least about 70° C. for about 24 hours. TABLE 1 Initial Initial Penetration Resistance Alpha After Penetration Resistance Initial Alpha After Heat After Heat Heat Sample 1-1 3.83 1.93 73.28 9 1.91 54.64 Sample 1-2 3.5 2.44 59.7 5.08 2.47 52.36 Sample 1-3 59.4 0.281 80.6 58.36 0.36 65.33

As shown in Table 1, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as magnesium stearate, produced an electret polymer fiber web with enhanced properties. In particular, Sample 1-3 has an initial alpha value that is significantly higher than the initial alpha value of Samples 1-1 and 1-2, each of which contain only one of the two additives.

As is further shown in Table 1, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as magnesium stearate, also produced an electret polymer fiber web having an enhanced charge stability. After subjecting Samples 1-1, 1-2, and 1-3 to heat, the alpha value of Sample 1-3 decreased from 80.6 to 65.33, however the resulting alpha value of 65.33 was still significantly higher than the resulting alpha values of Samples 1-1 and 1-2. Sample 1-3 thus showed a high initial alpha value, and the ability to maintain an acceptable alpha value.

EXAMPLE 2

Sample 2-1 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99% by weight of polypropylene and 1% by weight of ACRAWAX™ C. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 6.8μ, and a web basis weight of 60.4 g/m².

Sample 2-2 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99.6% by weight of polypropylene and 0.4% by weight of magnesium stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 7.4μ, and a web basis weight of 60.04 g/m².

Sample 2-3 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 98.6% by weight of polypropylene, 1% by weight of ACRAWAX™ C, and 0.4% by weight of magnesium stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 3.4μ, and a web basis weight of 63.73 g/m².

Table 2 illustrates the penetration, the resistance at 10.5 fpm face velocity, and the alpha value of Samples 2-1, 2-2, and 2-3, as tested using a NaCl challenge, initially and after subjecting the webs to a temperature of at least about 70° C. for about 24 hours. TABLE 2 Initial Initial Penetration Resistance Alpha After Penetration Resistance Initial Alpha After Heat After Heat Heat Sample 2-1 9.82 1.58 63.8 18.25 1.64 45.02 Sample 2-2 20.3 1.87 37 22.7 1.97 32.77 Sample 2-3 0.06 6.74 47.98 0.07 6.38 49.03

As shown in Table 2, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as magnesium stearate, produced an electret polymer fiber web with enhanced properties. In particular, while Sample 2-3 has an initial alpha value that is lower than the initial alpha value of Sample 2-1, and that is not significantly higher than the initial alpha value of Sample 2-2, the alpha value was maintained, and in fact it even increased, after the Samples were subjected to heat. The alpha value of Sample 2-1 decreased substantially from 63.8 to 45.02. The initial alpha value of Sample 2-2 was only 37, and it decreased to 32.77 after being subjected to heat. Whereas the alpha value of Sample 2-3 was initially 47.98, and it increased to 49.03 after being subjected to heat. The resulting alpha value, after heat, of Sample 2-3 was higher than the resulting alpha value of Samples 2-1 and 2-2. Accordingly, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as magnesium stearate, produced an electret polymer fiber web having an enhanced charge stability.

EXAMPLE 3

Sample 3-1 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99% by weight of polypropylene and 1% by weight of ACRAWAX™ C. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 7.8%, and a web basis weight of 22.43 g/m².

Sample 3-2 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99.6% by weight of polypropylene and 0.4% by weight of zinc stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 7.4μ, and a web basis weight of 22 g/m².

Sample 3-3 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 98.6% by weight of polypropylene, 1% by weight of ACRAWAX™ C, and 0.4% by weight of zinc stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 3.4μ, and a web basis weight of 22.53 g/m².

Table 3 illustrates the penetration, the resistance at 10.5 fpm face velocity, and the alpha value of Samples 3-1, 3-2, and 3-3, as tested using a NaCl challenge, initially and after subjecting the webs to a temperature of at least about 70° C. for about 24 hours. TABLE 3 Initial Initial Penetration Resistance Alpha After Penetration Resistance Initial Alpha After Heat After Heat Heat Sample 3-1 55.97 0.44 57.41 65.89 0.4 45.75 Sample 3-2 69.8 0.341 45.8 74.74 0.38 33.01 Sample 3-3 2.99 2.29 66.67 8.39 2.12 50.74

As shown in Table 3, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as zinc stearate, produced an electret polymer fiber web with enhanced properties. In particular, Sample 3-3 has an initial alpha value that is significantly higher than the initial alpha value of Samples 3-1 and 3-2, each of which contain only one of the two additives.

As is further shown in Table 3, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as zinc stearate, also produced an electret polymer fiber web having an enhanced charge stability. After subjecting Samples 3-1, 3-2, and 3-3 to heat, the alpha value of Sample 3-3 decreased from 66.67 to 50.74, however the resulting alpha value of 50.74 was still significantly higher than the resulting alpha values of Samples 3-1 and 3-2. Sample 3-3 thus showed a high initial alpha value, and the ability to maintain an acceptable alpha value.

EXAMPLE 4

Sample 4-1 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99% by weight of polypropylene and 1% by weight of ACRAWAX™ C. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 3.6μ, and a web basis weight of 61.93 g/m².

Sample 4-2 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 99.6% by weight of polypropylene and 0.4% by weight of zinc stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 3.6μ, and a web basis weight of 61.4 g/m².

Sample 4-3 of a meltblown electret polymer fiber web was prepared, as previously described, from a composition containing 98.6% by weight of polypropylene, 1% by weight of ACRAWAX™ C, and 0.4% by weight of zinc stearate. The meltblown electret polymer fiber web was prepared with fibers having a diameter of 8.71μ, and a web basis weight of 60.9 g/m².

Table 4 illustrates the penetration, the resistance at 10.5 fpm face velocity, and the alpha value of Samples 4-1, 4-2, and 4-3, as tested using a NaCl challenge, initially and after subjecting the webs to a temperature of at least about 70° C. for about 24 hours. TABLE 4 Initial Initial Penetration Resistance Alpha After Penetration Resistance Initial Alpha After Heat After Heat Heat Sample 4-1 0.14 5.44 52.59 1.18 5.07 38.03 Sample 4-2 0.064 6.75 47.3 0.19 7.23 37.77 Sample 4-3 15.4 1.25 65 16.06 1.32 60.17

As shown in Table 4, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as zinc stearate, produced an electret polymer fiber web with enhanced properties. In particular, Sample 4-3 has an initial alpha value that is significantly higher than the initial alpha value of Samples 4-1 and 4-2, and the alpha value was substantially maintained after the Sample were subjected to heat. The alpha value of Sample 4-1 decreased substantially from 52.59 to 38.03, and the alpha value of Sample 4-2 decreased from 47.3 to 37.77. Whereas the alpha value of Sample 4-3 was initially 65, and it only decreased to 60.17 after being subjected to heat. Accordingly, the combination of a fatty acid amide, such as ACRAWAX™ C, with a metal stearate, such as zinc stearate, produced an electret polymer fiber web having an enhanced charge stability.

One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety. 

1. A filter media, comprising a meltblown electret polymer fiber web having a melt processable fatty acid amide present within the web and effective to enhance the level of electrostatic charge, and a melt processable fatty acid metal salt present within the web and effective to maintain the electrostatic charge when the filter media is subjected to heat.
 2. The filter media of claim 1, wherein the melt processable fatty acid amide is present within the web at a concentration in the range of about 0.5% to 11% by weight.
 3. The filter media of claim 1, wherein the melt processable fatty acid amide is present within the web at a concentration in the range of about 1% to 8% by weight.
 4. The filter media of claim 1, wherein the melt processable fatty acid amide is present within the web at a concentration of about 1% by weight.
 5. The filter media of claim 1, wherein the melt processable fatty acid metal salt is present within the web at a concentration in the range of about 0.1% to 20% by weight.
 6. The filter media of claim 1, wherein the melt processable fatty acid metal salt is present within the web at a concentration in the range of about 0.1% to 5% by weight.
 7. The filter media of claim 1, wherein the melt processable fatty acid metal salt is present within the web at a concentration of about 0.4% by weight.
 8. The filter media of claim 1, wherein the melt processable fatty acid metal salt comprises a metal stearate.
 9. The filter media of claim 8, wherein the metal stearate comprises magnesium stearate.
 10. The filter media of claim 8, wherein the metal stearate comprises zinc stearate.
 11. The filter media of claim 1, wherein the melt processable fatty acid amide is selected from the group consisting of a stearamide, ethylene bis-stearamide, and mixtures thereof.
 12. The filter media of claim 1, wherein the meltblown electret polymer fiber web is formed from a polymer selected from the group consisting of polyethylene, polypropylene, polyamide, polyvinyl chloride, polymethylmethacrylate, polyester, and mixtures thereof.
 13. The filter media of claim 1, wherein the meltblown electret polymer fiber web is formed from fibers having a fiber diameter in the range of about 1μ to 15μ.
 14. The filter media of claim 1, wherein the meltblown electret polymer fiber web is formed from fibers having a fiber diameter of about 3μ.
 15. The filter media of claim 1, wherein the meltblown electret polymer fiber web has a basis weight that is in the range of about 10 g/m² to 200 g/m².
 16. The filter media of claim 1, wherein the meltblown electret polymer fiber web has a basis weight that is in the range of about 20 g/m² to 70 g/m².
 17. The filter media of claim 1, wherein the meltblown electret polymer fiber web is incorporated into a respiratory filter.
 18. A filter media, comprising an electret web of fibers, the fibers comprising a blend of a polymer, a fatty acid amide, and a metal stearate, the fatty acid amide being present within the web at a concentration in the range of about 0.5% to 11% by weight, and the metal stearate being present within the web at a concentration in the range of about 0.1% to 20% by weight.
 19. The filter media of claim 18, wherein the meltblown web is incorporated into a respiratory filter.
 20. A filter media, comprising a meltblown electret polymer fiber web having a basis weight of about 60 g/m and having a melt processable ethylene bis-stearamide present within the web at a concentration in the range of about 0.5% to 11% by weight, and a melt processable magnesium stearate present within the web at a concentration in the range of about 0.1% to 20% by weight.
 21. A meltblown electret web of polymer fibers, the polymers fibers comprising a polymer, about 0.5% to 11% by weight of a fatty acid amide, and about 0.1% to 20% by weight of a fatty acid metal salt, the fatty acid amide and the fatty acid metal salt being dispersed in the polymer as a melt blend additive.
 22. A method for forming a filter media, comprising: extruding a polymer composition containing a polymer, a fatty acid amide, and a fatty acid metal salt to form a meltblown polymer fiber web; and imparting an electric charge to the meltblown polymer fiber web to form a meltblown electret polymer fiber web.
 23. The method of claim 22, further comprising subjecting the meltblown electret polymer fiber web to a heat treatment.
 24. The method of claim 23, wherein the heat treatment comprises subjecting the meltblown electret polymer fiber web to a temperature of at least about 70° C.
 25. The method of claim 22, wherein a corona charge is used to impart the charge.
 26. The method of claim 22, wherein the polymer composition includes about 0.5% to 11% by weight of the melt processable fatty acid amide, and about 0.1% to 20% by weight of the melt processable fatty acid metal salt.
 27. The method of claim 22, wherein the polymer composition includes about 1% to 8% by weight of the melt processable fatty acid amide, and about 0.1% to 5% by weight of the melt processable fatty acid metal salt.
 28. The method of claim 22, wherein the polymer composition includes about 1% by weight of the melt processable fatty acid amide, and about 0.4% by weight of the melt processable fatty acid metal salt.
 29. The method of claim 22, wherein the melt processable metal salt comprises a metal stearate.
 30. The method of claim 29, wherein the metal stearate comprises magnesium stearate.
 31. The method of claim 29, wherein the metal stearate comprises zinc stearate.
 32. The method of claim 22, wherein the melt processable fatty acid amide is selected from the group consisting of a stearamide, ethylene bis-stearamide, and mixtures thereof. 